* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

.container {
  display: grid;
  grid-template-columns: repeat(3, 100px);
  grid-template-rows: repeat(3, 100px);
  
  /* 单元格对于网格线的对齐方式 */
  align-items: center;
  justify-items: center;

  /* 整个内容区域对于容器的对齐方式 */
  align-content: center;
  justify-content: center;
  
  width: 600px;
  height: 600px;
  margin: 100px auto;
  border: 1px solid red;
}

.item {
  width: 50px;
  height: 50px;
  text-align: center;
  line-height: 50px;
  border: 1px solid skyblue;
}
